An electron beam generating apparatus is provided with a window material used to emit an electron beam from a vacuum container outwardly. For example, Patent Document 1 discloses an irradiation window of an electron beam irradiation apparatus having a window material (window foil). FIG. 12 illustrates a structure of this irradiation window. In the irradiation window 100, a window foil 101 is placed between a grid window 102 having an opening through which electrons “e” are allowed to pass and a foil retaining plate 103, and is fixed by bolts 104. A gap between the window foil 101 and the grid window 102 is sealed with an O ring 105. The grid window 102 is held by a window holder 106. The window holder 106 is attached to a vacuum chamber 108 by bolts 107. A space between the window holder 106 and the vacuum chamber 108 is sealed with an O ring 109. A space between the foil retaining plate 103 and the window holder 106 is sealed with an elastic packing 110.
Patent Document 1: Japanese Published Unexamined Patent Application No. H9-203800